The present invention relates to pressure sensing transducers. More particularly, the present invention relates to pressure sensing transducers of the piezoresistive type.
Piezoresistive pressure transducers have a wide range of applications including industrial and other applications where accurate pressure monitoring is required. Typical industrial applications include process monitoring, rotating machinery monitoring and testing, oil exploration, jet engine and gas turbine engine controls, etc. Piezoresistive pressure transducers offer many potential advantages in such applications due to their small size, absence of moving parts and potential for sensitivity and accuracy.
The heart of a piezoresistive pressure transducer is a pressure force collector diaphragm having one or more piezoresistive elements mounted thereon. The diaphragm with the piezoresistive elements is typically placed in a pressure cell of some type which maintains a low pressure or vacuum on one side of the force collector diaphragm and allows the external medium under pressure to contact the other side of the diaphragm. A voltage is placed across the piezoresistive element(s) and as the diaphragm bends in response to pressure changes, a resistance change in the resistive element(s) results in a change in the current flowing through the resistive element(s).
In one prior art approach, a stainless steel force collector diaphragm is employed with piezoresistive foils bonded or otherwise mounted on the diaphragm. For example, a foil of nickel-chrome alloy may be bonded onto the stainless steel diaphragm. Such foil bonded pressure transducers have a number of disadvantages, however. In particular, the repeated flexing of the diaphragm in response to pressure changes will result in slippage of the bonded foil against the diaphragm. This will ultimately degrade the accuracy of the pressure transducer. Also, the sensitivity of such foil bonded transducers is quite low. Furthermore, the sensitivity of foil bonded pressure transducers to temperature variations is quite severe, thereby limiting the effective temperature range over which such transducers can give accurate pressure readings.
Another prior art approach employs a semiconductive piezoresistive material, typically silicon, sputtered or otherwise deposited in a thin film on a stainless steel force collector diaphragm. The silicon film is doped with a suitable concentration of dopant, typically a "P" type dopant, to result in a desired resistivity for the film. To electrically insulate the semiconductor film from the force collector diaphragm, an oxide layer is typically employed between the semiconductor material and the steel diaphragm.
Such silicon-on-stainless steel transducers have significant advantages over the foil bonded type transducers. In particular, silicon has a much higher piezoresistive response to deformations, approximately fifty times greater than typical foil type piezoresistive materials, thereby providing a transducer of correspondingly greater sensitivity. As previously mentioned, to insulate the thin film of semiconductor material from the stainless steel force collector diaphragm and prevent shorting problems, an oxide layer is typically employed between the silicon film and steel diaphragm. However, since three distinct types of materials are bonded together, each having differing crystal structures, this type of pressure transducer has inherent hysteresis effects which degrade accuracy over time. Also, the impossibility of matching the thermal expansion characteristics of silicon with those of the steel diaphragm and oxide layer results in inherent limitations and inaccuracies where large temperature variations are involved.
In another prior art approach, doped silicon piezoresistive elements are epitaxially grown directly on a force collector diaphragm of single crystal silicon. Since the silicon piezoresistive film is grown directly onto the silicon diaphragm, the piezoresistive film is essentially an atomic extension of the diaphragm and has the same crystal structure. This results in better bonding and effectively no hysteresis effect. Additionally, since the piezoresistive effect is dependent upon the orientation of the crystal structure of the silicon, piezoresistive films having different orientations may be formed on the diaphragm and an output provided which varies as the difference between the resistive values of the piezoresistors. Specifically, a Wheatstone bridge configuration of silicon piezoresistive elements may be laid out on the diaphragm (using well known doping, masking and etching techniques), thereby effectively amplifying the sensitivity of the piezoresistive elements to the force applied to the diaphragm.
Although having many advantages, such silicon-on-silicon transducers also have a number of inherent disadvantages as well. Since the silicon diaphragm is a semiconductor by nature, shorting of the piezoresistive elements through the silicon diaphragm may occur. To avoid this problem, each silicon piezoresistive element is typically formed in an island of oppositely doped conductivity type; for example, a P-type silicon piezoresistive element is formed in an N-type region which is in turn either formed on the diaphragm or doped into the silicon crystal of the diaphragm. The junction between the two conductivity types is then reverse biased to prevent current flow from the piezoresistive film into the diaphragm. As is well known, however, the reverse biased PN junction is temperature dependent in its characteristics. This results in inherent limitations on the operating temperature range of the transducer, with a practical upper limit of about 250.degree.-350.degree. F. Also, the use of silicon as a pressure collector diaphragm has inherent limitations for high pressure applications due to the limited strength of single crystal silicon. Additionally, the formation of the PN junction on the force collector diaphragm reduces the mechanical strength of the transducer and increases the manufacturing cost of the transducer.
Attempts have been made to overcome a number of the foregoing problems by employing a sapphire force collector diaphragm with silicon piezoresistive elements epitaxially formed thereon. Since sapphire is an electrical insulator there is no need for a reverse biased semiconductor junction between the piezoresistive elements and the diaphragm. Also, sapphire has a mechanical strength much greater than silicon and approximately 30% greater than even a stainless steel diaphragm. Furthermore, since the silicon crystal structure is compatible with that of sapphire, a single integral crystal structure may be formed by epitaxial growth of the silicon piezoresistive film on the sapphire diaphragm, thereby gaining the benefits of little or no hysteresis and crystallographic orientation dependent piezoresistive effects which are possessed by the silicon-on-silicon transducers.
Despite the promising nature of silicon-on-sapphire pressure transducers, however, to the best of applicant's knowledge, due to problems with matching the thermal expansion characteristics of silicon and sapphire, and other problems associated with the silicon-on-sapphire composite, no operational silicon-on-sapphire pressure transducers have been produced within the prior art. Accordingly, a need presently exists for an improved piezoresistive type pressure transducer having a high degree of accuracy throughout a wide pressure and temperature range.